The Supernova spotted last year was actually a shredded star

Until last year, astronomersbelieved that what they saw on late 2015 was the biggest and brightest supernova ever registered. However, after months of continuous observations, it was determined that a black hole that ripped apart a massive star caused the explosion, and it was not a supernova.

Back in 2015, the scientists from the All Sky Automated Survey for SuperNovae (ASAS-SN) thought that they saw one of the brightest supernovae humans have ever perceived. The so-called supernova was twice as bright as the previous one recorded. In fact, it shined about 20 times brighter than the total amount of light exit that comes from the entire Milky Way Galaxy, composed of over 100 billion stars.

Back in 2015, the scientists from the ASAS-SN thought that they saw one of the brightest supernovae humans have ever perceived. Photo credit: ESO, ESA / Hubble, M. Kornmesser / LCO

Initially, the dubbed ASASSN-15lh was categorized as a supernova. This cosmological event consists on the gigantic explosion of a massive star during their last stellar stage of evolution. This explosion marks the beginning of a smaller star that will fade over the course of days or months.

However, after months of observation and analysis, astronomers defined the phenomenon as a tidal disruption. This event is produced as a consequence of a supermassive black hole that is spinning very fast, and a Sun-like star that passes too close. This closeness makes the black hole destroy the star.

Often, when a star is too close to a black hole, this one devours the star instead of ripping it apart. In this particular scenario, the black hole was spinning so fast that destroyed the star immediately. Then, all the debris and heat that was produced because of the collision resulted in the giant burst of light that appeared to be a supernova.

The study and the theory

Astronomers from several observatories came to this conclusion this Monday, after months of investigation. The European Southern Observatory (ESO), UC Santa Barbara (UCSB) and other European and American institutes worked together in the publication that was presented in the first edition of the journal Nature Astronomy.

This brand new conclusion is sustained by images collected from distinct investigation centers. Some of these were not available when the first observation was made in late 2015, at the ASAS-SN. Images from the Paranal Observatory, La Silla Observatory, and NASA’s Hubble were gathered to prove this theory.

“Years ago we just wouldn’t have been able to follow an event like this. This study shows that large-area surveys, a global robotic telescope network and a NASA satellite can come together to reveal dramatic new discoveries that wouldn’t be possible without each piece of that puzzle,” said co-author Andy Howell, leader of the supernova group at the LCO and an adjunct professor in UCSB’s Department of Physics.

For over ten months, the investigators tried to discover why this phenomenon was so unique. Several reasons suggested that this event was not a supernova. For example, according to the investigation team leader, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel, and the Dark Cosmology Centre in Denmark Giorgos Leloudas, this event happened in a reddish galaxy, a place where this kind of things are not supposed to happen. He said that this type of galaxies doesn’t have the presence of massive stars that can produce superluminous supernovae.

After further observation, Leloudas and his team questioned the evolution of the explosion over time. They explained that this evolution is not consistent, and instead of presenting lower temperatures like supernovae do after some time, it started to get hotter. Three months after the explosion, the unique object warmed and stayed in an unseen high temperature, and even keeps that temperature at the present moment, an unexpected situation for the scientists.

In addition to these discoveries, the research team, thanks to images from the NASA’s Hubble, found that this object, the ASASSN-15lh, was located in the center of its galaxy. According to past studies made regarding the operation mechanisms of supermassive black holes, it was concluded that these cosmological structures were located in the center of their respective galaxies.

This black hole is gigantic

The investigation group discovered that this supermassive black hole measures about 200 million to 3 billion times more than our Sun. In relation to this, any black hole with this mass is supposed to swallow any star that passes near its core. However, this black hole behaved differently.

The Columbian University astronomer Nicholas Stone explained that it takes a body of particular characteristics for this event to take place. He said that even if the scientific world is sure about this phenomenon being a tidal disruption, is a unique one.

“The tidal disruption event we propose cannot be explained with a non-spinning supermassive black hole. We argue that ASASSN-15lh was a tidal disruption event arising from a very particular kind of black hole,” he stated in an interview this Tuesday.

Through history, there have been ten tidal disruption events registered, and according to Leloudas, the field of investigation concerning this phenomenon seems to be much more diverse than they thought. The research team isn’t fully confident on a tidal disruption as a cause for the ASASSN-15lh. However, they believe that the evidence points to that direction. Further investigation has to be made to define exactly what happened 4 billion light-years from Earth.